1. Field of the Invention
The present invention relates to canisters temporally trapping fuel vapor and being disposed on gasoline vehicles or the like.
2. Description of the Related Art
A gas vehicle is generally equipped with a fuel vapor processor for preventing fuel vapor vaporized in a fuel tank from flowing into the atmosphere. The fuel vapor processor includes a canister filled with an adsorbent and temporally trapping the fuel vapor by adsorbing the fuel vapor onto the adsorbent. As for a conventional canister shown in Japanese Laid-Open Patent Publication No. 2006-138290, the canister has a housing provided with an introducing pipe, an air communicating pipe and an exhaust pipe. The introducing pipe is connected to a fuel tank via a solenoid valve, which can prevent communication between the canister and the fuel tank. The air communicating pipe communicates the canister with the atmosphere. The exhaust pipe is connected to an engine via a vacuum switching valve, which can prevent communication between the engine and the canister. The solenoid valve and the vacuum switching valve are controlled by an electric control unit (ECU).
The housing of the canister has a partition for preventing gas from flowing in the canister along a shortcut route between introducing pipe and the exhaust pipe. The housing divides its inner space into a first adsorption chamber including a first and a second compartments, and a second adsorption chamber. The first adsorption chamber communicates with the second adsorption chamber, a first compartment and a second compartment, which directly communicate with the air communicating pipe, the introducing pipe and the exhaust pipe, respectively. The first and the second adsorption chamber and the second compartment are filled with activated carbons as adsorbent, whereas the first compartment is empty.
The activated carbon filled in the second compartment is composed of a crushed activated carbon, and the first and the second adsorption chamber are filled with a granulated activated carbon having a larger diameter than the crushed activated carbon.
When refueling the fuel tank, the solenoid valve is opened, and then the fuel gas including fuel vapor and air in the fuel tank flows through the introducing pipe and the first compartment and into the first adsorption chamber. Most of the fuel vapor in the fuel gas adsorbs onto the activated carbon in the first adsorption chamber, whereas the fuel vapor that is not trapped in the first adsorption chamber flows into the second adsorption chamber and then is trapped by the activated carbon in the second adsorption chamber. The remaining air flows through the second adsorption chamber and the air communicating pipe and then into the atmosphere.
During driving or in a high pressure condition of the fuel tank, the fuel gas generated in the fuel tank flows through the introducing pipe, the first compartment, the second compartment, the exhaust pipe and the vacuum switching valve and then into the engine. In such condition, ambient air flows through the air communicating pipe and into the canister so that the fuel vapor adsorbed onto the activated carbons in the canister are detached from the activated carbon and flows into the engine (so-called purge operation).
Because the first compartment is empty, airflow resistance (pressure drop) in the canister on a first route between the air communicating pipe and the exhaust pipe is larger than one on a second route between the introducing pipe and the exhaust pipe. Therefore, the amount of gas flowing in the canister along the second route is likely to be larger than the amount of gas flowing along the first route during the purge operation. Thus, there is a need for improved canisters.